Ophthalmologic imaging apparatus and ophthalmologic image processing apparatus

ABSTRACT

An ophthalmologic imaging apparatus that can follow up imaging for acquiring a cross sectional image by referring to a front image of an eye acquired in the past and scanning the same position as before with light, includes: a photographing part configured to photograph the eye and acquire a front image thereof; a cross sectional image forming part configured to scan the eye with light and form a cross sectional image thereof; a storage configured to store a first front image of the eye and a second front image acquired in follow up imaging executed with referring to the first front image; an information obtaining part configured to analyze the first and second front images and obtain misregistration information between these front images; and a calculator configured to calculate an evaluation value of an error in a scanning position in the follow up imaging based on the misregistration information.

TECHNICAL FIELD

The present invention relates to an ophthalmologic imaging apparatus andan ophthalmologic image processing apparatus.

BACKGROUND TECHNOLOGY

In recent years, optical coherence tomography (OCT) has attractedattention as an apparatus for imaging an eye using optical scanning.Unlike an X-ray CT apparatus, optical coherence tomography isnoninvasive to human bodies, and is therefore expected to be utilized inthe medical field and biological field. For example, in theophthalmology, apparatuses that form images of a fundus and cornea orthe like are in a practical stage.

The apparatus disclosed in Patent Document 1 uses a technique ofso-called “Fourier Domain OCT.” That is to say, the apparatus irradiatesa low-coherence light beam to an object, superposes the reflected lightand the reference light to generate an interference light, and acquiresthe spectral intensity distribution of the interference light to executeFourier transform, thereby imaging the morphology in the depth direction(the z-direction) of the object. Furthermore, the apparatus is providedwith a galvano mirror that scans a light beam (signal light) along onedirection (x-direction) perpendicular to the z-direction, and is therebyconfigured to form an image of a desired measurement target region ofthe object. An image formed by this apparatus is a two-dimensional crosssectional image in the depth direction (z-direction) along the scanningdirection (x-direction) of the light beam. The technique of this type isalso called Spectral Domain.

Patent Document 2 discloses a technique of scanning with a signal lightin the horizontal direction (x-direction) and the vertical direction(y-direction) to form multiple two-dimensional cross sectional images inthe horizontal direction, and acquiring and imaging three-dimensionalcross sectional information of a measured range based on the crosssectional images. As the three-dimensional imaging, for example, amethod of arranging and displaying multiple cross sectional images inthe vertical direction (referred to as stack data or the like), or amethod of executing a rendering process on volume data (voxel data)based on stack data to form a three-dimensional image may be considered.

Patent Documents 3 and 4 disclose other types of OCT apparatuses. PatentDocument 3 describes an OCT apparatus that images the morphology of anobject by sweeping the wavelength of light that is irradiated to anobject (wavelength sweeping), detecting interference light obtained bysuperposing the reflected lights of the light of the respectivewavelengths on the reference light to acquire its spectral intensitydistribution, and executing Fourier transform. Such an OCT apparatus iscalled a Swept Source type or the like. The Swept Source type is a kindof the Fourier Domain type.

Further, Patent Document 4 describes an OCT device that irradiates alight having a predetermined beam diameter to an object and analyzes thecomponents of an interference light obtained by superposing thereflected light and the reference light, thereby forming an image of theobject in a cross-section orthogonal to the travelling direction of thelight. Such an OCT device is called a full-field type, en-face type orthe like.

Patent Document 5 discloses an example of applying OCT to theophthalmologic field. It should be noted that, before OCT was applied, aretinal camera, a slit lamp microscope, a scanning laser opthalmoscope(SLO) etc. were used as apparatuses for observing an eye (see PatentDocuments 6, 7 and 8 for example). The retinal camera is an apparatusthat photographs the fundus by projecting illumination light onto theeye and receiving the reflected light from the fundus. The slit lampmicroscope is an apparatus that obtains an image of the cross-section ofthe cornea by cutting off the light section of the cornea using slitlight. The SLO is an apparatus that images morphology of retinal surfaceby scanning a fundus by laser light and detecting reflected lightthereof with a highly sensitive imaging element such as aphotomultiplier.

The apparatus with OCT is superior relative to the retinal camera, etc.in that high-definition images can be obtained, further in that crosssectional images and three-dimensional images can be obtained, etc.

Thus, the apparatus using OCT can be used for observation of variousregions of the eye and is capable of obtaining high-definition images,and therefore, has been applied to the diagnosis of variousophthalmologic disorders.

Now, in various medical fields, examinations of same sites of a subjectare repeated (sometimes referred to as follow up). Examples of follow upinclude progress observations and preoperative/postoperativeobservations. There are specific problems in ophthalmology regardingfollow up. Specifically, it is difficult to examine a same siterepeatedly because of influences of eye movement (involuntary eyemovement during fixation, rotation, etc.) of a subject eye. Inparticular, reproducing the same scanning position as a past examinationis extremely difficult for an imaging apparatus of the type that scansan eye with light.

Patent Documents 9 to 11 discloses technologies that can be applied forsolving such a problem of follow up imaging.

The purpose of an invention disclosed in Patent Document 9 is tosmoothly detect misregistration (including rotational transfer) betweenfundus images. For this purpose, this invention is configured to detectan amount of misregistration between fundus images by collating aplurality of subregion images cut from the respective fundus images.

The purpose of an invention disclosed in Patent Document 10 is topreferably detect retinal endogenous signals of a human eye bycontinuously measuring the same cross sectional site of a fundus. Forthis purpose, this invention is configured to detect displacements ofscanning positions by irradiating and detecting a second light beamwhile scanning the fundus with a measuring light beam and control asecond optical scanner based on the detection results, therebycorrecting the scanning positions of the measuring light beam asrequired. Further, it is also described that template matching based oncharacteristic regions in retinal front images is utilized for detectingmisregistration of scanning positions.

The purpose of an invention disclosed in Patent Document 11 is tocorrectly irradiate a light beam to a preset position on a fundusregardless of eye movement. For this purpose, this invention isconfigured to detect picture angle information of scanning light andmisregistration information of the fundus based on fundus front imagesand correct scanning positions of scanning light based on theseinformation.

PRIOR ART DOCUMENTS Patent Documents

[Patent Document 1]

Japanese Unexamined Patent Application Publication No. H11-325849

[Patent Document 2]

Japanese Unexamined Patent Application Publication No. 2002-139421

[Patent Document 3]

Japanese Unexamined Patent Application Publication No. 2007-24677

[Patent Document 4]

Japanese Unexamined Patent Application Publication No. 2006-153838

[Patent Document 5]

Japanese Unexamined Patent Application Publication No. 2008-73099

[Patent Document 6]

Japanese Unexamined Patent Application Publication No. H09-276232

[Patent Document 7]

Japanese Unexamined Patent Application Publication No. 2008-259544

[Patent Document 8]

Japanese Unexamined Patent Application Publication No. 2009-11381

[Patent Document 9]

Japanese Unexamined Patent Application Publication No. 2011-50430

[Patent Document 10]

Japanese Unexamined Patent Application Publication No. 2011-115507

[Patent Document 11]

Japanese Unexamined Patent Application Publication No. 2011-212213

SUMMARY OF THE INVENTION Problem that the Invention is to Solve

In follow up imaging, it is very important to realize a degree of anerror (accuracy, precision), that is, a degree of misregistration ofscanning positions between imagings carried out different timings.Therefore, in order to carry out diagnosis precisely, it is veryimportant to know whether or not follow up imaging has carried outproperly, and further, degrees of accuracy and precision of the followup imaging. However, according to conventional technologies, it has beenimpossible to quantitatively evaluate degrees of errors in follow upimaging.

For example, the technology disclosed in Patent Document 9 may detectthe amount of misregistration between fundus images; however, it isimpossible to provide, for users, meanings of misregistration andinformation indicating whether or not the follow up imaging isappropriate. Thus, the user has to judge by himself based on themisregistration detected. However, it is difficult to judge whether ornot the follow up imaging is appropriate since errors of scanning startposition in follow up imaging is usually of millimeter order or smaller.Further, there are various types of misregistration such as parallelshifts and rotational shifts, and it has been impossible to evaluatemisregistration at least from the perspective ofappropriateness/inappropriateness of follow up imaging.

Further, technologies disclosed in Patent Documents 10 and 11 detectmisregistration during imaging and correct scanning positions. However,even though such correction is carried out, there still remainsmisregistration of scanning positions, that is, errors in follow upimaging. These conventional technologies cannot quantitatively evaluatesuch errors in follow up imaging.

The present invention is developed in order to solve such problems, andits purpose is to provide a technology that is capable of quantitativelyevaluating degrees of errors in follow up imaging.

Means for Solving the Problem

An aspects the present invention is an ophthalmologic imaging apparatuscapable of carrying out follow up imaging for acquiring a crosssectional image by referring to a front image of an eye acquired in thepast and scanning the same position as before with light, comprising: aphotographing part configured to photograph the eye and acquire a frontimage thereof; a cross sectional image forming part configured to scanthe eye with light and form a cross sectional image thereof; a storageconfigured to store a first front image of the eye and a second frontimage acquired in follow up imaging carried out with referring to thefirst front image; an information obtaining part configured to analyzethe first and second front images and obtain misregistration informationbetween these front images; and a calculator configured to calculate anevaluation value of an error in a scanning position in the follow upimaging based on the misregistration information.

Effect of the Invention

According to the present invention, it is possible to quantitativelyevaluate degrees of errors in follow up imaging.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram showing an example of a configuration ofan ophthalmologic imaging apparatus according to an embodiment.

FIG. 2 is a schematic diagram showing an example of a configuration ofan ophthalmologic imaging apparatus according to an embodiment.

FIG. 3 is a schematic block diagram showing an example of aconfiguration of an ophthalmologic imaging apparatus according to anembodiment.

FIG. 4 is a schematic block diagram showing an example of aconfiguration of an ophthalmologic imaging apparatus according to anembodiment.

FIG. 5A is a schematic diagram for explaining an operation of anophthalmologic imaging apparatus according to an embodiment.

FIG. 5B is a schematic diagram for explaining an operation of anophthalmologic imaging apparatus according to an embodiment.

FIG. 5C is a schematic diagram for explaining an operation of anophthalmologic imaging apparatus according to an embodiment.

FIG. 5D is a schematic diagram for explaining an operation of anophthalmologic imaging apparatus according to an embodiment.

FIG. 6A is a schematic diagram for explaining an operation of anophthalmologic imaging apparatus according to an embodiment.

FIG. 6B is a schematic diagram for explaining an operation of anophthalmologic imaging apparatus according to an embodiment.

FIG. 6C is a schematic diagram for explaining an operation of anophthalmologic imaging apparatus according to an embodiment.

FIG. 6D is a schematic diagram for explaining an operation of anophthalmologic imaging apparatus according to an embodiment.

FIG. 7 is a schematic diagram for explaining an operation of anophthalmologic imaging apparatus according to an embodiment.

FIG. 8 is a schematic diagram for explaining an operation of anophthalmologic imaging apparatus according to an embodiment.

FIG. 9 is a schematic block diagram showing an example of aconfiguration of an ophthalmologic imaging apparatus according to anembodiment.

FIG. 10 is a schematic block diagram showing an example of aconfiguration of an ophthalmologic imaging apparatus according to anembodiment.

FIG. 11 is a schematic block diagram showing an example of aconfiguration of an ophthalmologic image processing apparatus accordingto an embodiment.

MODE FOR CARRYING OUT THE INVENTION

Examples of embodiments of an ophthalmological imaging apparatus andophthalmological image processing apparatus according to the presentinvention will be described in detail with reference to the drawings.The ophthalmologic imaging apparatus according to the present inventionforms a cross sectional image and three-dimensional image of an eyefundus by using OCT. In this description, images obtained by OCT aresometimes referred to as OCT images. Further, a measurement operationfor forming an OCT image is sometimes referred to as OCT measurement. Itshould be noted that the contents described in the documents cited inthis description may be applied to the following embodiments.

In the following embodiments, configurations in which Fourier Domain OCTis employed will be described in detail. Particularly, ophthalmologicimaging apparatuses according to the following embodiments are capableof obtaining both a fundus OCT image with Spectral Domain OCT and afundus image, which is similar to the apparatus disclosed in PatentDocument 5. It should be noted that configurations according to thepresent invention may be applied to an ophthalmologic imaging apparatusof any type other than Spectral Domain (for example, Swept Source OCT).

Further, although the following embodiments include a retinal camera asa configuration for acquiring front images of an eye, other imagingapparatus may be applied such as an SLO, slit lamp microscope,ophthalmologic operation microscope, etc. It should be noted that frontimages of an eye are images acquired by photographing a desired site ofthe eye (fundus, anterior eye part, etc.) from a viewpoint confrontingthe eye.

Further, an ophthalmological image processing apparatus according to thepresent invention may be configured as a part of an ophthalmologicalimaging apparatus, may be configured as a single computer, or may beconfigured as two or more computers connected via a network.

First Embodiment Configurations

An ophthalmologic imaging apparatus 1, as shown in FIG. 1 and FIG. 2,includes a retinal camera unit 2, an OCT unit 100, and an arithmetic andcontrol unit 200. The retinal camera unit 2 has almost the same opticalsystem as a conventional retinal camera. The OCT unit 100 is providedwith an optical system for obtaining an OCT image of a fundus. Thearithmetic and control unit 200 is provided with a computer thatexecutes various arithmetic processes, control processes, and so on.

[Retinal Camera Unit]

The retinal camera unit 2 shown in FIG. 1 is provided with an opticalsystem for forming a 2-dimensional image (fundus image) representing thesurface morphology of the fundus Ef of an eye E. Fundus images includeobservation images, photographed images, etc. The observation image is,for example, a monochromatic moving image formed at a prescribed framerate using near-infrared light. The photographed image may be, forexample, a color image captured by flashing visible light, or amonochromatic still image captured by using near-infrared light orvisible light as illumination light. The retinal camera unit 2 may alsobe configured to be capable of capturing other types of images such as afluorescein angiography image, an indocyanine green fluorescent image,and an autofluorescent image. It should be noted that a fundus image ofany type acquired by the retinal camera unit 2 corresponds to a “frontimage”. Further, the retinal camera unit 2 corresponds to a“photographing part”.

The retinal camera unit 2 is provided with a chin rest and a foreheadplacement for supporting the face of the subject. Moreover, the retinalcamera unit 2 is provided with the illumination optical system 10 andthe imaging optical system 30. The illumination optical system 10irradiates illumination light to the fundus Ef. The imaging opticalsystem 30 guides a fundus reflected light of the illumination light toimaging devices (CCD image sensors 35, 38 (sometimes referred to simplyas CCD)). Moreover, the imaging optical system 30 guides signal lightinput from the OCT unit 100 to the fundus Ef, and guides the signallight returned from the fundus Ef to the OCT unit 100.

An observation light source 11 of the illumination optical system 10comprises, for example, a halogen lamp. Light (observation illuminationlight) output from the observation light source 11 is reflected by areflection mirror 12 with a curved reflection surface, and becomesnear-infrared after passing through a visible cut filter 14 via acondenser lens 13. Furthermore, the observation illumination light isonce converged near an imaging light source 15, reflected by a mirror16, and passes through relay lenses 17 and 18, a diaphragm 19, and arelay lens 20. Then, the observation illumination light is reflected onthe peripheral part (the surrounding region of an aperture part) of anaperture mirror 21, transmitted through a dichroic mirror 46, andrefracted by an object lens 22, thereby illuminating the fundus Ef. Itshould be noted that LED (Light Emitting Diode) may be used as theobservation light source.

The fundus reflection light of the observation illumination light isrefracted by the object lens 22, transmitted through the dichroic mirror46, passes through the aperture part formed in the center region of theaperture mirror 21, transmitted through a dichroic mirror 55, travelsthrough a focusing lens 31, and reflected by a mirror 32. Furthermore,the fundus reflection light is transmitted through a half-mirror 39A,refracted by reflected by a dichroic mirror 33, and forms an image onthe light receiving surface of the CCD image sensor 35 by a condenserlens 34. The CCD image sensor 35 detects the fundus reflection light ata preset frame rate, for example. An image (observation image) based onthe fundus reflection light detected by the CCD image sensor 35 isdisplayed on a display device 3. It should be noted that when theimaging optical system is focused on the anterior eye part, theobservation image of the anterior eye part of the eye E is displayed.

The imaging light source 15 comprises, for example, a xenon lamp. Thelight (imaging illumination light) output from the imaging light source15 is irradiated to the fundus Ef via the same route as that of theobservation illumination light. The fundus reflection light of theimaging illumination light is guided to the dichroic mirror 33 via thesame route as that of the observation illumination light, transmittedthrough the dichroic mirror 33, reflected by a mirror 36, and forms animage on the light receiving surface of the CCD image sensor 38 by acondenser lens 37. An image (photographed image) based on the fundusreflection light detected by the CCD image sensor 38 is displayed on thedisplay device 3. It should be noted that the display device 3 fordisplaying the observation image and the display device 3 for displayingthe photographed image may be the same or different. Further, whensimilar photographing is carried out by illuminating the eye E withinfrared light, infrared photographed image is displayed. Moreover, LEDmay be used as the imaging light source.

An LCD (Liquid Crystal Display) 39 displays a fixation target or atarget for measuring visual acuity. The fixation target is a visualtarget for fixating the eye E, and is used when photographing a fundusor performing OCT measurement.

Part of the light output from the LCD 39 is reflected by the half-mirror39A, reflected by the mirror 32, passes through the aperture part of theaperture mirror 21, refracted by the object lens 22, and projected ontothe fundus Ef.

By changing a display position of the fixation target on the screen ofthe LCD 39, it is possible to change the fixation position of the eye E.Examples of the fixation positions of the eye E include the position foracquiring an image centered at the macula of the fundus Ef, the positionfor acquiring an image centered at the optic papilla, the position foracquiring an image centered at the fundus center located between themacula and the optic papilla, and so on, as in conventional retinalcameras. Further, the display position of the fixation target may bearbitrarily changed.

Furthermore, as with conventional retinal cameras, the retinal cameraunit 2 is provided with an alignment optical system 50 and a focusoptical system 60. The alignment optical system 50 generates a target(alignment target) for matching the position (alignment) of the deviceoptical system with respect to the eye E. The focus optical system 60generates a target (split target) for matching the focus with respect tothe eye Ef.

Light (alignment light) output from an LED 51 of the alignment opticalsystem 50 passes through diaphragms 52 and 53 and a relay lens 54, isreflected by the dichroic mirror 55, passes through the aperture part ofthe aperture mirror 21, is transmitted through the dichroic mirror 46,and is projected onto the cornea of the eye E by the object lens 22.

Cornea reflection light of the alignment light passes through the objectlens 22, the dichroic mirror 46 and the aperture part, a part of thecornea reflection light is transmitted through the dichroic mirror 55,passes through the focusing lens 31, reflected by the mirror 32,transmitted through the half-mirror 39A, reflected by the dichroicmirror 33, and projected onto the light receiving surface of the CCDimage sensor 35 by the condenser lens 34. An image (alignment target)captured by the CCD image sensor 35 is displayed on the display device 3together with the observation image. The user conducts alignment by anoperation that is the same as conventional retinal cameras. Further,alignment may be performed in a way in which the arithmetic and controlunit 200 analyzes the position of the alignment target and controls themovement of the optical system (automatic alignment function).

In order to conduct focus adjustment, the reflection surface of areflection rod 67 is positioned at a slanted position on the opticalpath of the illumination optical system 10. Light (focus light) outputfrom an LED 61 of the focus optical system 60 passes through a relaylens 62, is split into two light fluxes by a split target plate 63,passes through a two-hole diaphragm 64, is reflected by a mirror 65, andis reflected after an image is formed once on the reflection surface ofthe reflection rod 67 by a condenser lens 66. Furthermore, the focuslight passes through the relay lens 20, is reflected at the aperturemirror 21, is transmitted through the dichroic mirror 46, is refractedby the object lens 22, and is projected onto the fundus Ef.

The fundus reflection light of the focus light passes through the sameroute as the cornea reflection light of the alignment light and isdetected by the CCD image sensor 35. An image (split target) captured bythe CCD image sensor 35 is displayed on the display device 3 togetherwith the observation image. The arithmetic and control unit 200, as inthe conventional technology, analyzes the position of the split target,and moves the focusing lens 31 and the focus optical system 60 forfocusing (automatic focusing function). Further, focusing may beperformed manually while visually recognizing the split target.

The dichroic mirror 46 splits the optical path for OCT from the opticalfor eye fundus photographing. The dichroic mirror 46 reflects light ofthe wavelength band used for OCT, and transmits the light for eye fundusphotographing. The optical path for OCT is provided with a collimatorlens unit 40, an optical path length changing part 41, a galvano scanner42, a focusing lens 43, a mirror 44 and a relay lens 45.

The optical path length changing part 41 is capable of moving in thedirection indicated by the arrow in FIG. 1 to change the length of theoptical path for OCT. This change of optical path length may be used forcorrection of the optical path length in accordance with the axiallength of the eye E, and for adjustment of the condition ofinterference. The optical path length changing part 41 is configured tocomprise a corner cube and a mechanism for moving the corner cube, forexample.

The galvano scanner 42 changes the travelling direction of light (signallight LS) travelling along the optical path for OCT. Thereby, the fundusEf is scanned by the signal light LS. The galvano scanner 42 isconfigured to comprise a galvano mirror for scanning with the signallight LS in the x-direction, a galvano mirror for scanning in they-direction, and a mechanism for independently driving these. Thereby,the signal light LS may be scanned in an arbitrary direction in thexy-plane.

[OCT Unit]

An example of the configuration of the OCT unit 100 is explained whilereferring to FIG. 2. The OCT unit 100 is provided with an optical systemfor obtaining an OCT image of the fundus Ef. The optical systemcomprises a similar configuration to a conventional Spectral Domain OCTapparatus. That is to say, this optical system is configured to splitlow-coherence light into signal light and reference light, superpose thesignal light returned form the fundus Ef and the reference light havingtraveled through a reference optical path to generate interferencelight, and detect the spectral components of the interference light.This detection result (detection signal) is transmitted to thearithmetic and control unit 200.

It should be noted that when Swept Source OCT apparatus is used, a sweptsource is provided instead of a low-coherence light source while anoptical member for spectrally decomposing interference light is notprovided. In general, any known technology in accordance with the typeof OCT may be arbitrarily applied for the configuration of the OCT unit100.

A light source unit 101 outputs broadband low-coherence light L0. Thelow-coherence light L0, for example, includes near-infrared wavelengthband (about 800-900 nm) and has a coherence length of about tens ofmicrometer. Moreover, it is possible to use, as the low-coherence lightL0, near-infrared light having wavelength band that is impossible to bedetected by human eyes, for example, infrared light having the centerwavelength of about 1040-1060 nm.

The light source unit 101 is configured to comprise light output device,such as an SLD (super luminescent diode), LED, SOA (SemiconductorOptical Amplifier) and the like.

The low-coherence light L0 output from the light source unit 101 isguided to a fiber coupler 103 by an optical fiber 102 and split into thesignal light LS and the reference light LR.

The reference light LR is guided to an optical attenuator 105 by anoptical fiber 104. Through any known technology, the optical attenuator105 received control of the arithmetic and control unit 200 forautomatically adjusting light amount (light intensity) of the referencelight LR guided to the optical fiber 104. The reference light LR havingadjusted by the optical attenuator 105 is guided to a polarizationcontroller 106 by the optical fiber 104. The polarization controller 106is a device configured to, for example, apply stress to the loop-formoptical fiber 104 from outside to adjust polarization condition of thereference light LR being guided in the optical fiber 104. It should benoted that the configuration of the polarization controller 106 is notlimited to this, and arbitrary known technology may be applied. Thereference light LR whose polarization condition has been adjusted by thepolarization controller 106 is guided to an optical coupler 109.

The signal light LS generated by the fiber coupler 103 is guided by theoptical fiber 107, and converted into a parallel light flux by thecollimator lens unit 40. Further, the signal light LS travels throughthe optical path length changing part 41, the galvano scanner 42, thefocusing lens 43, the mirror 44 and the relay lens 45, and reaches thedichroic mirror 46. Further, the signal light LS is reflected by thedichroic mirror 46, refracted by the objective lens 22, and projected tothe fundus Ef. The signal light LS is scattered (including reflection)at various depth positions of the fundus Ef. The back-scattered light ofthe signal light LS from the fundus Ef travels along the same route asthe outward way in the opposite direction to the fiber coupler 103, andis reached the fiber coupler 109 through an optical fiber 108.

The fiber coupler 109 superposes the back-scattered light of the signallight LS and the reference light LR having passed through the opticalfiber 104. Interference light LC thus generated is guided by an opticalfiber 110 and output from an exit end 111. Furthermore, the interferencelight LC is converted into a parallel light flux by a collimator lens112, spectrally divided (spectrally decomposed) by a diffraction grating113, converged by a condenser lens 114, and projected onto the lightreceiving surface of a CCD image sensor 115. It should be noted thatalthough the diffraction grating 113 shown in FIG. 2 is of transmissiontype, any other kind of a spectrally decomposing element (such asreflection type) may be used.

The CCD image sensor 115 is for example a line sensor, and detects therespective spectral components of the spectrally decomposed interferencelight LC and converts the components into electric charges. The CCDimage sensor 115 accumulates these electric charges, generates adetection signal, and transmits the detection signal to the arithmeticand control unit 200.

Although a Michelson-type interferometer is employed in the presentembodiment, it is possible to employ any type of interferometer such asa Mach-Zehnder-type as necessary. Instead of a CCD image sensor, othertypes of image sensors, such as a CMOS (Complementary Metal OxideSemiconductor) image sensor, may be used.

[Arithmetic and Control Unit]

A configuration of the arithmetic and control unit 200 will bedescribed. The arithmetic and control unit 200 analyzes the detectionsignals input from the CCD image sensor 115 to form an OCT image of thefundus Ef. Arithmetic processing for this may be the same as that of aconventional Spectral Domain OCT apparatus.

Further, the arithmetic and control unit 200 controls each part of theretinal camera unit 2, the display device 3 and the OCT unit 100. Forexample, the arithmetic and control unit 200 displays an OCT image ofthe fundus Ef on the display device 3.

Further, as controls of the retinal camera unit 2, the arithmetic andcontrol unit 200 executes: controls of actions of the observation lightsource 101, the imaging light source 103 and LED's 51 and 61; control ofaction of the LCD 39; controls of movements of the focusing lenses 31and 43; control of movement of the reflection rod 67; control ofmovement of the focus optical system 60; control of movement of theoptical path length changing part 41; control of action of the galvanoscanner 42; and so on.

Further, as controls of the OCT unit 100, the arithmetic and controlunit 200 executes: control of action of the light source unit 101;control of action of the optical attenuator 105; control of action ofthe polarization controller 106; control of action of the CCD imagesensor 115; and so on.

The arithmetic and control unit 200 comprises a microprocessor, a RAM, aROM, a hard disk drive, a communication interface, and so on, as inconventional computers. The storage device such as the hard disk drivestores a computer program for controlling the ophthalmologic imagingapparatus 1. The arithmetic and control unit 200 may be provided withvarious circuit boards such as a circuit board for forming OCT images.Moreover, the arithmetic and control unit 200 may be provided withoperation devices (input devices) such as a keyboard, a mouse, etc.and/or a display device such as an LCD etc.

The retinal camera unit 2, the display device 3, the OCT unit 100, andthe arithmetic and control unit 200 may be integrally configured (thatis, provided within a single case), or separately configured in two ormore cases.

[Control System]

A configuration of a control system of the ophthalmologic imagingapparatus 1 will be described with reference to FIGS. 3 and 4.

Now, the ophthalmologic imaging apparatus 1 is configured to be capableof executing follow up imaging and post-processing thereof. The followup imaging is an imaging technique for acquiring a cross sectional imageby referring to a front image of an eye acquired in the past andscanning the same position as before with light. Further, thepost-processing includes processing of evaluating an error in scanningpositions of light in the follow up imaging. In other words, thepost-processing includes processing of evaluating the gap(misregistration) between a scanning position in the past imaging and ascanning position in the follow up imaging based on a front imageacquired in this past imaging. It should be noted that theophthalmologic image processing apparatus according to embodimentscarries out such post-processing.

Before explaining the control system, follow up imaging will beexplained briefly. In follow up imaging, controls of respective parts ofthe ophthalmologic imaging apparatus 1 are executed by the controller210 (in particular, the main controller 211).

Follow up imaging is an imaging method of reproducing a scanningposition applied in the past imaging and performing the present imaging.In a preparatory phase thereof, a patient selecting screen (illustrationomitted) is displayed on the display 240A. The patient selecting screenis provided with: a function for selecting a patient who becomes asubject of follow up imaging (patient selecting part); a function fordisplaying patient information (patient information display); and afunction for displaying imaging information (imaging informationdisplay). Further, the patient selecting screen is provided with variousoperation parts (software keys).

The patient selecting part includes a space for inputting a retrievalquery, calendar for selecting a date of imaging (such as the date of thelast imaging in the past), etc. Once information is input into thepatient selecting part, the controller 210 searches patient informationstored in an information storage (the storage 212 of the ophthalmologicimaging apparatus 1, database on a network, etc.), and displays thesearch result on the patient information display. Such informationdisplayed may include a patient ID, patient name, sex, date of birth,etc. It should be noted that when a date of imaging is selected from thecalendar and the like, a plurality of patient information are listed inthe patient information display. In such a case, the user may select adesired patient from among the plurality of patient information.

Once one patient is selected, the controller 210 obtains imaginginformation relating to this patient and displays it on the imaginginformation display. If imaging has been carried out a plurality oftimes in the past, a plurality of imaging information relating to theplurality of imaging are listed in chronological order. The imaginginformation may include the date of imaging, time of imaging, datastorage destination (file address etc.), information whether or not thisis follow up imaging, scanning mode of OCT measurement (describedlater), identification information of left/right eye(s), fixationposition, information relating to analysis (retinal thickness analysisetc.), and so on.

Once the user selects imaging information as a reference in the presentfollow up imaging, the controller 210 obtains image data acquired byimaging corresponding to the selected imaging information from theinformation storage, and displays images based on the image data (frontimage and cross sectional image) on the display 240A. Here, displayscreen is switched from the patient selecting screen to an image displayscreen (illustration omitted). The user observes the displayed imagesand determines whether to carry out follow up imaging by referring tothe front image. If other front images are referred to, other imagesacquired by this imaging or images acquired by other imaging aredisplayed and similar determination is carried out. Once the userdetermines a referred front image, the controller 210 displays thisfront image (reference front image, or first front image) on the display240A, and control status is transferred to waiting status for a commandof commencing follow up imaging.

Once a predetermined imaging-commencing command is input, the controller210 acquires near-infrared moving image of the eye E in real time. Then,the controller 210 executes image matching between frames of thisnear-infrared moving image (second front image, or follow-up frontimage) and the reference front image while performing tracking(described later), and executes OCT measurement with the same scanningmode as the past imaging. This OCT measurement is started at a timing atwhich tracking is carried out properly, for example. Further, scanningwith the same scanning mode is repeated a predetermined times in thisOCT measurement. An image forming part 220 averages multiple image dataacquired in this repeating scanning to form a final image.

Image data of OCT images acquired from such follow up imaging areassociated with patient information, scanning position information ofthis follow up imaging, frames of near-infrared moving image acquired inthis follow up imaging, imaging information of this follow up imaging,information relating to referred past imaging (scanning positioninformation, imaging information, image data, etc.), and stored in thestorage 212. Here, the frames of near-infrared moving image to be storedare frames acquired first and last in the period in which OCTmeasurement is carried out, for example.

This is the end of the explanation of processing executed in follow upimaging and explanation of the control system starts again.

(Controller)

The control system of the ophthalmologic imaging apparatus 1 has aconfiguration centered on a controller 210. The controller 210 isconfigured to comprise, for example, the aforementioned microprocessor,RAM, ROM, hard disk drive, and communication interface, etc. Thecontroller 210 is provided with a main controller 211 and storage 212.

(Main Controller)

The main controller 211 performs the aforementioned various kinds ofcontrols. Specifically, the main controller 211 controls a focus driver31A, the optical path length changing part 41 and the galvano scanner 42of the retinal camera unit 2, and further controls the light source unit101, the optical attenuator 105 and the polarization controller 106 ofthe OCT unit 100.

The focus driver 31A moves the focusing lens 31 in the direction of theoptical axis. Thereby, the focus position of the imaging optical system30 is changed. It should be noted that the main controller 211 maycontrol an optical system driver (not shown in diagrams) to threedimensionally move the optical system provided in the retinal cameraunit 2. This control is used for alignment and tracking. Tracking is anoperation for move the optical system in accordance with eye movement ofthe eye E. When tracking is applied, alignment and focusing are carriedout in advance. Tracking is a function to maintain adequate positionalrelationship in which alignment and focusing are matched by causing theposition of the optical system to follow the eye movement.

The main controller 211 executes a process of writing data into thestorage 212, and a process of reading out data from the storage 212. Themain controller 211 includes a display controller 2111 that displaysvarious information on the display 240A. The display controller 2111functions as an example of a “first display part” and “second displaypart”. Processing executed by the display controller 2111 is describedlater.

(Storage)

The storage 212 stores various kinds of data. The data stored in thestorage 212 may include image data of OCT images, image data of fundusimages, and eye information, for example. The eye information includesinformation on the eye, such as information on a subject such as apatient ID and a name, identification information on left eye or righteye, and so on. Further, the storage 212 stores various programs anddata for operating the ophthalmologic imaging apparatus 1.

The storage 212 stores follow up imaging information 2121. The follow upimaging information 2121 is information relating to follow up imagingcarried out in the past. The follow up imaging information 2121 includesat least: front images of the eye E referred to in follow up imagingcarried out in the past (first front image, or reference front image);and front images of the eye E acquired in this follow up imaging (secondfront image, or follow-up front image). The latter front images areframes of near-infrared moving image acquired in real time in thisfollow up imaging. The frames are frames acquired first and last in theperiod in which OCT measurement is carried out.

Further, the follow up imaging information 2121 may include referencescanning position information indicating scanning position (firstscanning position, or reference scanning position) of a cross sectionalimage formed together with the reference front image; and follow-upscanning position information indicating scanning position (secondscanning position, or follow-up scanning position) of a cross sectionalimage acquired together with the follow-up front image. These scanningposition information include control information of the galvano scanner42 in the scanning, that is, information indicating directions of thegalvano scanner 42, for example. Further, if front images are beingacquired in real time during OCT measurement, the scanning positioninformation may include coordinates of scanning position (scanninglocus) depicted in the front images.

(Image Forming Part)

The image forming part 220 forms image data of a cross sectional imageof the fundus Ef based on the detection signals from the CCD imagesensor 115. Like conventional Spectral Domain OCT, this process includesprocesses such as noise elimination (noise reduction), filtering and FFT(Fast Fourier Transform).

When scanning is carried out predetermined times in succession with thesame scanning mode, the image forming part 220 executes processing offorming new image data by superposing image data of multiple crosssectional images acquired from this successive scanning. Thissuperposition is carried out for the purpose of eliminating randomnoises mixed in image data.

In the case in which other OCT type is applied, the image forming part220 executes known processing in accordance with the applied OCT type.The image forming part 220 is configured to include the aforementionedcircuit board, for example. The image forming part 220 functions as a“cross sectional image forming part” together with an optical systemused for OCT measurement. It should be noted “image data” and an “image”based on the image data may be identified with each other in thedescription.

(Image Processor)

An image processor 230 executes various image processing and analysisprocessing on OCT images formed by the image forming part 220. Forexample, the image processor 230 executes various correction processingsuch as brightness correction, dispersion correction of images, etc.Moreover, the image processor 230 analyzes OCT images to perform layerthickness analysis for obtaining retinal thickness distribution.Further, the image processor 230 executes various image processing andanalysis processing on images obtained by the retinal camera unit 2(fundus images, anterior eye part images, etc.).

The image processor 230 executes known image processing such asinterpolation processing for interpolating pixels between crosssectional images to form image data of a three-dimensional image of thefundus Ef. It should be noted that the image data of a three-dimensionalimage refers to image data that the positions of pixels are defined bythe three-dimensional coordinates. The image data of a three-dimensionalimage is, for example, image data composed of three-dimensionallyarranged voxels. This image data is referred to as volume data, voxeldata, or the like. For displaying an image based on the volume data, theimage processor 230 executes a rendering process (such as volumerendering and MIP (Maximum Intensity Projection)) on this volume data,and forms image data of a pseudo three-dimensional image taken from aspecific view direction. On a display device such as a display 240A,this pseudo three-dimensional image is displayed.

Further, it is also possible to form stack data of multiple crosssectional images as the image data of a three-dimensional image. Stackdata is image data obtained by three-dimensionally arranging multiplecross sectional images obtained along multiple scanning lines, based onthe positional relation of the scanning lines. That is to say, stackdata is image data obtained by expressing multiple cross sectionalimages defined by originally individual two-dimensional coordinatesystems by a three-dimensional coordinate system (in other words,embedding into a three-dimensional space).

The image processor 230 includes an information obtaining part 231,calculator 232 and imaging-propriety judging part 233. They executepost-processing of follow up imaging. The post-processing evaluates anerror in scanning position of light in follow up imaging as describedabove.

(Information Obtaining Part)

The information obtaining part 231 analyzes a front image referred to infollow up imaging (reference front image) and a front image acquired inthe follow up image (follow-up front image) to obtain misregistrationinformation between these front images. The misregistration informationquantitatively expresses how much the depicted position of morphology ofthe fundus Ef in these front images is shifted.

As the misregistration information, the information obtaining part 231calculates a parallel shift error and rotational shift error between thereference front image and follow-up front image, for example. Theparallel shift error corresponds to a displacement, in a spreadingdirections of front images (that is, in the xy-plane), of morphology ofthe fundus Ef depicted in these front images. The rotational shift errorcorresponds to a displacement of the morphology in a rotationaldirection centered at a certain position in the xy-plane. It should benoted that the parallel shift error and rotational shift error areexpressed as an affine transformation between coordinates of both frontimages, for example. Further, it may be configured to calculate any oneof the parallel shift error and rotational shift error.

An example of method of calculating the parallel shift error androtational shift error is explained. Firstly, the information obtainingpart 231 analyzes the respective front images to specify image positionscorresponding to a predetermined characteristic site of the fundus Ef.The characteristic site is, for example, the center/edge of an opticdisk, center of a macula, specific blood vessel, branch point of a bloodvessel, lesion site, etc.

Next, the information obtaining part 231 obtains a displacement betweenthe coordinates of the image point in the reference front image and thecoordinates of the image point in the follow-up front image. Thisprocessing determines, for each of the image positions of multiplecharacteristic sites, components of an affine transformation matrix bysubstituting the coordinates of the image positions in the both frontimages into the known formula of two-dimensional affine transformation,for example. It should be noted that the coordinates of the imagepositions are address information assigned to the respective pixels inadvance. The affine transformation matrix obtained in this way includesboth information of the parallel shift error and rotational shift errorbetween the front images.

(Calculator)

The calculator 232 calculates an evaluation value of an error in ascanning position in follow up imaging based on the misregistrationinformation obtained by the information obtaining part 231. Examples ofthe evaluation value include: an evaluation value calculated based on anarea of a preset image region defined by the scanning position relatingto the reference front information and the scanning position relating tothe follow-up front information; and an evaluation value calculatedbased on a displacement between these scanning positions. It should benoted that an evaluation value may be calculated based on a factor(s)other than these. Further, an evaluation value may be calculated bycombining different factors.

The calculator 232 includes a relative position calculator 2321, judgingpart 2322, area calculator 2323, displacement calculator 2324 andevaluation value calculator 2325. Calculation of an evaluation valuebased on the area is carried out by the relative position calculator2321, judging part 2322 and area calculator 2323. Calculation of anevaluation value based on the displacement is carried out by therelative position calculator 2321, displacement calculator 2324 andevaluation value calculator 2325.

(Relative Position Calculator)

As described above, the follow up imaging information 2121 stored in thestorage 212 includes the reference scanning position information and thefollow-up scanning position information. The relative positioncalculator 2321 calculates relative position information between thereference scanning position information and the follow-up scanningposition information based on the misregistration information obtainedby the information obtaining part 231.

Now, follow up imaging is an imaging method in which a past scanningposition is reproduced and OCT measurement is carried out. Thus, in anideal case, the reference scanning position and the follow-up scanningposition coincide with each other. However, in reality, it is difficultto completely reproduce scanning position due to involuntary eyemovement during fixation etc. On the other hand, since the referencescanning position and the follow-up scanning position coincide with eachother if involuntary eye movement during fixation etc. is not involved,the relative position between these scanning positions corresponds tomisregistration (displacement) of the eye E due to involuntary eyemovement during fixation etc., that is, corresponds to misregistrationbetween the reference scanning position and the follow-up scanningposition. This misregistration is obtained as the misregistrationinformation by the information obtaining part 231.

The relative position calculator 2321, for example, displaces thereference scanning position by an amount of misregistration indicated inthe misregistration information to obtain a position corresponding tothe reference scanning position in the follow-up front image, that is,to obtain an ideal scanning position realized in a case in which thereference scanning position is completely reproduced. The relativeposition information indicates the relative position between this idealscanning position and the scanning position applied in the actual followup imaging (follow-up scanning position). In this way, the relativeposition information and the misregistration information aresubstantially equivalent. Specifically, the misregistration informationindicates a displacement between the front images while the relativeposition information indicates a displacement between the scanningpositions. The relative position calculator 2321 sends the obtainedrelative position information to the judging part 2322 and thedisplacement calculator 2324.

(Judging Part)

FIGS. 5A to 5D illustrate examples of aspects of the relative positionbetween the reference scanning position and the follow-up scanningposition. In these diagrams, a symbol R indicates the follow-up scanningposition in the follow-up front image. Further, a symbol R0 indicatesthe reference scanning position in the follow-up front image, that is,the reference scanning position displaced based on the misregistrationinformation (the relative position information). FIG. 5A indicates anexample of a case in which only the parallel shift error is involved.FIG. 5B indicates an example of a case in which only the rotationalshift error is involved and both scanning positions intersects with eachother near their centers. FIG. 5C indicates an example of a case inwhich only the rotational shift error is involved and both scanningpositions intersects with each other near their scanning startpositions. FIG. 5D indicates an example of a case in which both theparallel shift error and the rotational shift error are involved. Itshould be noted that aspects of relative positions of the scanningpositions are not limited to these. Further, scanning modes are notlimited to the line scan.

The judging part 2322 judges whether or not the reference scanningposition and the follow-up scanning position have a common positionbased on the misregistration information (the relative positioninformation). The common position indicates a position (a region) inwhich the reference scanning position and the follow-up scanningposition are overlapped with each other.

The common position is a zero-dimensional region (a point),one-dimensional region (a line), or two-dimensional region (a plane). InFIGS. 5A to 5D, since both of the reference scanning position and thefollow-up scanning position are of a line-segment shape, the commonposition becomes an intersection point when they intersect with eachother and no common position exist when they do not intersect. Further,although illustration is omitted, if the direction of displacementbetween the reference scanning position and the follow-up scanningposition is only the length direction and the amount of the displacementis less than the length, then the common position of these two scanningpositions becomes a one-dimensional region. Further, whenthree-dimensional scan is used as scanning mode, both of the referencescanning position and the follow-up scanning position becometwo-dimensional regions, and the common position of these becomes azero, one or two-region in accordance with relative positionalrelationship.

Judgment whether or not the common position exists is carried out basedon the coordinates of the reference scanning position and thecoordinates of the follow-up scanning position in the follow-up frontimage. For example, the judging part 2322 collates the coordinates ofthe reference scanning position with the coordinates of the follow-upscanning position, and judges the common position exists if the commoncoordinates exist between these scanning positions and judges the commonposition does not exist if the common coordinates does not exist. Fromthis processing, the coordinates of the common position in the follow-upfront image are also obtained. It should be noted that these coordinatesare coordinates in a two-dimensional coordinate system defined for thefollow-up front image.

The judging part 2322 sends the judgment result relating to the commonposition (presence/absence of the common position, coordinates of thecommon position, etc.) to the area calculator 2323.

(Area Calculator)

The area calculator 2323 calculates an area of an image region definedby the reference scanning position and the follow-up scanning positionbased on the misregistration information (relative positioninformation). Here, the misregistration information and the relativeposition information are substantially equivalent. Further, the imageregion whose area is to be calculated is a partial region of thefollow-up front image. Further, a method of defining the image regionfrom the two scanning positions is predetermined. Further, processing ofcalculating the area may be carried out in an arbitrary way. Theprocessing of calculating the area may include, for example, processingof counting the number of pixels in the image region, processing ofderiving a mathematical expression (an equation etc.) expressing theboundary of the image region, or integration, or the like. Further, thearea, that is the result of calculation, may be a numerical valueuniquely expressing the two-dimensional size of the image region.

The area calculator 2323 executes different arithmetic processing inaccordance with the judgment result from the judging part 2322. Forexample, the area calculator 2323 executes one arithmetic processingwhen the reference scanning position and the follow-up scanning positionhave the common position and another arithmetic processing when they donot. Hereinafter, specific examples thereof are explained. The followingspecific examples treat cases in which two scanning positions are of aline-segment shape, and consider cases in which these scanning positionsintersect and cases in which they do not separately. That is, cases inwhich the common position exists and cases in which it does not areconsidered separately. Further, regarding cases in which two scanningpositions intersect with each other, cases in which the intersectionpoint is located at an endpoint and cases in which it is located atother point are considered separately. Judgment whether or not theintersection point is located at an endpoint may be easily carried outbased on the coordinates of the two scanning positions and thecoordinates of the intersection point. It should be noted that theendpoints of the scanning position indicates the scanning start pointand the scanning end point of the scanning position of a line-segmentshape. Further, the scanning positions are of a line-segment shape andso there exists only one intersection point in the following specificexamples; however, when the scanning position is of a curved shape thereare cases in which two or more intersection points exist.

When two scanning positions intersect as indicated in FIGS. 5B and 5C,the area calculator 2323 calculates the area of the image region definedby the endpoints of the two scanning positions and the intersectionpoint. More specifically, the area calculator 2323 calculates a sum ofan area of a triangle formed by the intersection point and the endpoints of the two scanning positions located on one side from theintersection point and an area of a triangle formed by the intersectionpoint and the end points of the two scanning positions located on theother side.

For example, when the intersection point is located at a position otherthan the endpoints of the scanning positions as illustrated in FIG. 5B,the area calculator 2323 calculates, as shown in FIG. 6A, the area ofthe triangle TR1 having the endpoints (scanning end points) R0E and REof the two scanning positions R0 and R located on the right side of theintersection point C and the intersection point C as vertices thereof.Further, the area calculator 2323 calculates the area of the triangleTR2 having the endpoints (scanning start points) R0S and RS of the twoscanning positions R0 and R located on the left side of the intersectionpoint C and the intersection point C as vertices thereof. Then, the areacalculator 2323 adds the area of the triangle TR1 to the area of thetriangle TR2. This sum is an objective value of an area.

As another example, when the two scanning positions intersect at theendpoints (scanning start points) as illustrated in FIG. 5C, the areacalculator 2323 calculates the area of the triangle TR having theintersection point C (both scanning start points), the scanning endpoint of the reference scanning position R0 and the scanning end pointof the follow-up scanning position R as vertices thereof.

It should be noted that when the two scanning positions intersect at anendpoint of one scanning position and a point of the other scanningposition other than its endpoints, it is possible to calculate, forexample, the area of the triangle having the intersection point, theopposite endpoint of one scanning position and one endpoint of the otherscanning position (such as an endpoint located on the same side as thisopposite endpoint) as vertices thereof.

On the other hand, when two scanning positions do not intersect asillustrated in FIGS. 5A and 5D, the area calculator 2323 calculates thearea of a quadrangle defined by the two scanning positions and sets itas the target area of image region. More specifically, when two scanningpositions do not intersect, the area calculator 2323 calculates, asillustrated in FIGS. 6C and 6D, the area of a quadrangle QU having thereference scanning position R0 and the follow-up scanning position R astwo sides and two line segments connecting endpoints of these scanningpositions as other two sides.

The area calculator 2323 sends the area obtained in the above fashion tothe evaluation value calculator 2325.

(Displacement Calculator)

The displacement calculator 2324 calculates a displacement between thereference scanning position and the follow-up scanning position based onthe misregistration information (the relative position information).Ways of calculating the displacement are arbitrary. Hereinafter,specific examples of the displacement calculation are explained withreferring to FIG. 7.

FIG. 7 illustrates the reference scanning position R0 and the follow-upscanning position R, each of which is of a line-segment shape. It shouldbe noted that calculation of the displacement may be carried outregardless of the fact whether or not the two scanning positionsintersect.

The displacement calculator 2324 firstly specifies predeterminedpositions on the respective scanning positions R0 and R. Thisspecification processing may be carried out based on the coordinates ofthe respective points of the scanning positions R0 and R of aline-segment shape. Examples of the predetermined position include ascanning start point, scanning end point, middle point, etc. It shouldbe noted that when scanning positions of a shape other than line-segmentshape are considered, it is possible to apply in accordance with theshape. Further, predetermined positions of the same type may be appliedfor two scanning positions which are to be compared. For example, when ascanning start point is used as the predetermined position, bothscanning start points may be specified. Further, when multiple types ofpredetermined positions are specified, the respective predeterminedpositions are associated with its type.

Next, the displacement calculator 2324 calculates the displacementbetween the coordinates of the predetermined position in the referencescanning position R0 and the coordinates of the predetermined positionin the follow-up scanning position R. This calculation may be carriedout by using a two-dimensional coordinate system previously defined forthe follow-up front image. Further, the displacement may be calculatedby counting the number of pixels located between the two predeterminedpositions. The displacement calculated here is a displacement betweenthe predetermined positions the same type. For example, thisdisplacement is a displacement between two scanning start points and nota displacement between a scanning start point and a middle point.

FIG. 7 illustrates a case in which three types (scanning start points,scanning end points and middle points) are applied as the abovementionedpredetermined positions. In FIG. 7, a symbol DS indicates a displacementbetween the scanning start point R0S of the reference scanning positionR0 and the scanning start point RS of the follow-up scanning position R.Further, a symbol DE indicates a displacement between the scanning endpoint R0E of the reference scanning position R0 and the scanning endpoint RE of the follow-up scanning position R. Further, a symbol DMindicates a displacement between the middle point R0M of the referencescanning position R0 and the middle point RM of the follow-up scanningposition R.

The displacement calculator 2324 sends the displacement calculated insuch a way to the evaluation value calculator 2325.

(Evaluation Value Calculator)

The evaluation value calculator 2325 calculates an evaluation value ofan error in the scanning position applied in the follow up imaging. Theinformation of area obtained by the area calculator 2323 and theinformation of displacement obtained by the displacement calculator 2324are input into the evaluation value calculator 2325 as information usedfor this calculation. Hereinafter, an example of calculating anevaluation value based on the area, an example of calculating anevaluation value based on the displacement, and an example ofcalculation that is a combination of both are described.

It should be noted that the present embodiment describes a configurationcapable of executing both calculation methods of evaluation values;however, a configuration capable of any one of these may be employed.

(Calculation of Evaluation Values Based on Area)

As an example of calculating an evaluation value from the area of theimage region calculated by the area calculator 2323, the evaluationvalue calculator 2325 may executes arithmetic processing of subtractinga product of the area of the image region and a preset weight from apreset maximum of evaluation values. This arithmetic processing may beexpressed as the following equation, for example.S ₁ =S _(1,max) −a*(Area)  [Equation 1]

Here, “S₁” indicates an evaluation value based on the area, “S_(1,max)”indicates a preset maximum of the concerned evaluation value, “a”indicates a preset weight, and “Area” indicates the area calculated bythe area calculator 2323. The maximum S_(1,max) and the weight a may bearbitrarily set. For example, the maximum S_(1,max) is set to 100 andthe weight a is set based on the magnitudes of this maximum and thearea, and the like.

According to such arithmetic processing, the smaller the area is (thatis, the smaller the error of the scanning position in the follow upimaging is), the greater the evaluation value S₁ becomes.

(Calculation of Evaluation Values Based on Displacement)

As an example of calculating an evaluation value from the displacementcalculated by the displacement calculator 2324, the evaluation valuecalculator 2325 may executes arithmetic processing of subtracting aproduct of the displacement and a preset weight from a preset maximum ofevaluation values. This arithmetic processing may be expressed as thefollowing equation, for example.S ₂ =S _(2,max)−(b*DM+c*DS+d*DE)  [Equation 2]

Here, “S₂” indicates an evaluation value based on the displacement,“S_(2,max)” indicates a preset maximum of the concerned evaluationvalue, “b”, “c” and “d” indicate preset weights, and “DM”, “DS” and “DE”indicate the displacements of middle points, scanning start points andscanning end points calculated by the displacement calculator 2324,respectively. The maximum S_(2,max) and the weights b, c and d may bearbitrarily set. For example, the maximum S_(2,max) is set to 100 andthe weights b, c and d are set based on the magnitudes of this maximumand the displacement, and the like.

According to such arithmetic processing, the smaller the displacement is(that is, the smaller the error of the scanning position in the followup imaging is), the greater the evaluation value S₂ becomes. It shouldbe noted that the evaluation value is calculated by taking three typesof points (middle points, scanning start points and scanning end points)into consideration in this example; however, the evaluation value may becalculated by taking one or two of these into consideration. Further, itis possible to calculate a displacement of points other than these andtake it into consideration.

(Calculation of Evaluation Values Based on Area and Displacement)

As an example of calculating an evaluation value based on the area ofthe image region calculated by the area calculator 2323 and thedisplacement calculated by the displacement calculator 2324, theevaluation value calculator 2325 may utilize an arithmetic expressionthat is obtained by combining the respective cases described above. Thisarithmetic expression may be given by the following, for example.S=S _(max) −a*(Area)−(b*DM+c*DS+d*DE)  [Equation 3]

Here, “S” indicates an evaluation value based on the area anddisplacement, “S_(max)” indicates a preset maximum of the concernedevaluation value, “a” to “d” indicate preset weights, “Area” indicatesthe area calculated by the area calculator 2323, and “DM”, “DS” and “DE”indicate the displacements of middle points, scanning start points andscanning end points calculated by the displacement calculator 2324,respectively. The maximum S_(max) and the weights a to d may bearbitrarily set. For example, the maximum S_(max) is set to 100 and theweights a to d are set based on the magnitudes of this maximum and thearea, and the like.

According to such arithmetic processing, the smaller the area and/or thedisplacement are/is (that is, the smaller the error of the scanningposition in the follow up imaging is), the greater the evaluation valueS becomes.

Information of the evaluation value calculated by the evaluation valuecalculator 2325 is sent to the imaging-propriety judging part 233.

(Imaging-Propriety Judging Part)

The imaging-propriety judging part 233 judges propriety of follow upimaging based on the evaluation value calculated by the evaluation valuecalculator 2325. This processing may be carried out by comparing theevaluation value with a preset numerical value range.

Specific examples of processing executed by the imaging-proprietyjudging part 233 are described. In a case in which decrease of an errorcauses increase of evaluation value as described above, theimaging-propriety judging part 233 judges whether or not the calculatedevaluation value is equal to or greater than a preset threshold. Whenthe evaluation value is equal to or greater than the threshold, theimaging-propriety judging part 233 judges that the follow up imaging hasbeen performed properly. Conversely, when the evaluation value issmaller than the threshold, the imaging-propriety judging part 233judges that the follow up imaging has not been performed properly.

It should be noted that although only one threshold is used in thepresent example, two or more thresholds may be used to judge thepropriety of follow up imaging in steps.

The imaging-propriety judging part 233 sends the judgment result to thecontroller 210. Further, any information used in the above processingexecuted by the image processor 230 and information intermediatelygenerated in the above processing may be sent to the controller 210.Further, in a case in which the propriety of follow up imaging ispresented by using the evaluation value itself, the imaging-proprietyjudging part 233 may not be provided.

The image processor 230 that functions as above comprises, for example,the aforementioned microprocessor, RAM, ROM, hard disk drive, circuitboard, and so on. A computer program that causes the microprocessor toperform the above functions is stored in the storage device such as thehard disk drive in advance.

(User Interface)

A user interface 240 comprises the display 240A and the operation part240B. The display 240A is configured to include a display device of theaforementioned arithmetic and control unit 200 and/or the display device3. The operation part 240B is configured to include an operation deviceof the aforementioned arithmetic and control unit 200. The operationpart 240B may also comprise various kinds of buttons, keys, etc. thatare provided with the case of the ophthalmologic imaging apparatus 1 oroutside thereof. For example, when the retinal camera unit 2 has a casethat is similar to conventional retinal cameras, a joy stick, operationpanel, etc. provided with the case may also be included in the operationpart 240B. Furthermore, the display 240A may also include variousdisplay devices such as a touch panel etc. provided with the case of theretinal camera unit 2.

The display 240A and the operation part 240B do not need to beconfigured as separate components. For example, like a touch panel, itis possible to apply a device in which the display function and theoperation function are integrated. In this case, the operation part 240Bis configured to include the touch panel and a computer program. Acontent of operation to the operation part 240B is input into thecontroller 210 as an electrical signal. Further, operations and/orinformation input may be carried out by using a graphical user interface(GUI) displayed on the display 240A and the operation part 240B.

[Scanning with Signal Light and OCT Images]

Scanning modes of the signal light LS by the ophthalmologic imagingapparatus 1 may include, for example, horizontal scan, vertical scan,cross scan, radial scan, circular scan, concentric scan, helical scan,etc. These scanning modes are selectively used as necessary taking intoaccount an observation site of a fundus, an analysis target (retinalthickness etc.), time required for scanning, the density of scanning,and so on.

The horizontal scan is one for scanning the signal light LS in thehorizontal direction (x-direction). The horizontal scan includes a modeof scanning the signal light LS along multiple scanning lines extendingin the horizontal direction arranged in the vertical direction(y-direction). In this mode, the interval of scanning lines may bearbitrarily set. Further, by setting the interval between adjacentscanning lines to be sufficiently narrow, it is possible to form theaforementioned three-dimensional image (three-dimensional scan). Thevertical scan is performed in a similar manner.

The cross scan is one for scanning the signal light LS along across-shape trajectory consisting of two linear trajectories (linetrajectories) orthogonal to each other. The radial scan is one forscanning the signal light LS along a radial trajectory consisting ofmultiple line trajectories arranged at predetermined angles. It shouldbe noted that the cross scan is an example of the radial scan.

The circular scan is one for scanning the signal light LS along acircular trajectory. The concentric scan is one for scanning the signallight LS along multiple circular trajectories arranged concentricallyaround a predetermined center position. The circular scan is an exampleof the concentric scan. The helical scan is one for scanning the signallight LS along a helical trajectory while making the turning radiusgradually smaller (or greater).

Since the galvano scanner 42 is configured to scan the signal light LSin the directions orthogonal to each other, the galvano scanner 42 iscapable of scanning the signal light LS in the x-direction and they-direction independently. Moreover, it is possible to scan the signallight LS along an arbitrary trajectory on the xy-plane by simultaneouslycontrolling the directions of two galvano mirrors included in thegalvano mirror 42. Thus, various kinds of scanning modes as describedabove may be realized.

By scanning the signal light LS in the modes described above, it ispossible to obtain a cross sectional image in the plane spanned by thedirection along the scanning line and the depth direction (z-direction)of the fundus. Moreover, in a case in which the interval betweenscanning lines is narrow, it is possible to obtain the aforementionedthree-dimensional image.

(Display Control)

Examples of display control in the present embodiment are described.Display control is executed by the display controller 2111.

As a first display control, the display controller 2111 displays thereference front image and/or the follow-up front image on the display240A and displays, over the front image(s), a reference scanningposition image indicating the reference scanning position and afollow-up scanning position image indicating the follow-up scanningposition based on the misregistration information obtained by theinformation obtaining part 231.

Here, instead of the misregistration information, the relative positioninformation that is equivalent to it may be referred to. Further, thereference scanning position image is an example of a “first scanningposition image” and the follow-up scanning position image is an exampleof a “second scanning position image”.

When any one of the reference front image and the follow-up front imageis to be displayed, the reference scanning position image and thefollow-up scanning position image are displayed over this front image.On the other hand, when both of the reference front image and thefollow-up front image are to be displayed, the reference scanningposition image and the follow-up scanning position image are displayedover the respective front images or over only one front image.

The display controller 2111 may display the reference scanning positionimage and the follow-up scanning position image in different aspectsfrom each other. For example, the two scanning position images may bedisplayed in different colors, different thicknesses, or differentdensities. Further, identification information (character stringinformation, image information, etc.) may be attached to the respectivescanning position images. Further, the two scanning position images maybe displayed in different aspects all times, or the display aspects maybe changed in response to a certain trigger.

FIG. 8 illustrates an example of information displayed by the firstdisplay control as described above. FIG. 8 illustrates a state in whichthe reference scanning position image (shown by a dotted line) and thefollow-up scanning position image (shown by a solid line) are displayedover a follow-up front image expressing the morphology of the fundus.The follow-up scanning position image indicates the scanning positionactually applied in the follow up imaging. Further, the referencescanning position image indicates an ideal scanning position with theassumption that the follow up imaging has been carried out with noerrors. In the example shown in FIG. 8, the site indicated by the solidline has been scanned due to eye movement etc. although the siteindicated by the dotted line should have been scanned.

As a second display control, the display controller 2111 displaysinformation indicating the error in the scanning positions in the followup imaging. For example, the display controller 2111 may display theevaluation value calculated by the evaluation value calculator 2325 onthe display 240A. The evaluation value may be displayed individually, ordisplayed together with front image(s) and/or display information fromthe above first display control. Further, instead of the evaluationvalue or together with the evaluation value, the judgment result fromthe imaging-propriety judging part 233 may be displayed. This judgmentresult may be character string information such as “proper” or“improper”, or image information.

[Actions and Effects]

Actions and effects of the ophthalmologic imaging apparatus 1 areexplained.

The ophthalmologic imaging apparatus 1 is capable of carrying out followup imaging. A photographing part (retinal camera unit 2) photographs theeye E and acquires a front image thereof. The cross sectional imageforming part (optical system for OCT and image forming part 220) scansthe eye E with light and forms a cross sectional image thereof. Thestorage 212 stores a first front image of the eye E and a second frontimage acquired in follow up imaging carried out with referring to thefirst front image. The information obtaining part (231) analyzes thefirst and second front images and obtains misregistration informationbetween these front images. The calculator (232) calculates anevaluation value of an error in a scanning position in the follow upimaging based on the misregistration information. According to theophthalmologic imaging apparatus 1, the degree of the error in thefollow up imaging can be evaluated quantitatively.

Second Embodiment

Follow up imaging is carried out by referring to a reference front imageacquired in the past. So, accuracy and precision of follow up imagingare influenced by the condition of the reference front image. In thepresent embodiment, an ophthalmologic imaging apparatus, in addition toarbitrary configuration described in the first embodiment, capable ofjudging propriety of a reference front image used in follow up imagingis described.

FIG. 9 illustrates a configuration example of the ophthalmologic imagingapparatus according to the present embodiment. The ophthalmologicimaging apparatus includes a candidate image judging part 234 inaddition to the configuration of the first embodiment (see FIGS. 1 to4). The candidate image judging part 234 is provided in the imageprocessor 230. It should be noted that descriptions of configurationsother than the candidate image judging part 234 are omitted unlessotherwise stated since they are the same as the first embodiment.

When follow up imaging is carried out, the storage 212 stores one ormore images (candidate images) to be a candidate for a reference frontimage. The candidate image judging part 234 analyzes the respectivecandidate images and judges whether or not the respective candidateimages are suitable for the reference front image. It should be notedthat when two or more candidate images are judged to be suitable, mostsuitable candidate image may be selected, for example.

Such judgment processing is carried out based on predeterminedinformation relating to candidate images, for example. This informationmay be information relating to a real space or information relating to afrequency space. Examples of the information relating to the real spaceinclude flare amount and contrast. Examples of the relating to thefrequency space include blur (dim) amount and frequency characteristic.Such information is obtained by the candidate image judging part 234.

Flare amount is a distribution of pixels of a candidate image havingpixel values (luminance values etc.) greater than a preset threshold(maximum etc.), for example. This distribution may be a ratio of suchpixels to whole pixels, for example. The candidate image judging part234 judges that a candidate image is suitable for a reference frontimage when the flare amount obtained is equal to or less than a presetthreshold, for example. Here, it is judged that the smaller the flareamount is, the more suitable for a reference image.

Contrast is obtained by arbitrary known technology executed based onpixel values (luminance values) of a candidate image, for example. Thecandidate image judging part 234 judges that a candidate image issuitable for a reference front image when the contrast obtained is equalto or greater than a preset threshold, for example. Here, it is judgedthat the greater the contrast is, the more suitable for a referenceimage.

Blur amount may be calculated based on signal intensities of spatialfrequency components obtained by decomposing a candidate image intospatial frequencies. The candidate image judging part 234 judges that acandidate image is suitable for a reference front image when the bluramount obtained is equal to or less than a preset threshold, forexample. Here, it is judged that the smaller the blur amount is, themore suitable for a reference image.

Frequency characteristic may be obtained by decomposing a candidateimage into spatial frequencies and calculating a characteristic of thespatial frequency components obtained. The candidate image judging part234 judges that a candidate image is suitable for a reference frontimage when the frequency characteristic obtained satisfies a presetcondition, for example. Here, it is judged that the higher the degree ofsatisfaction of the condition is, the more suitable for a referenceimage.

The controller 210 executes informing control based on the judgmentresult from the candidate image judging part 234. The controller 210 anda part controlled in the informing control function as an example of an“informing part”.

As a specific example of informing control, the display controller 2111displays information indicating the judgment result on the display 240A.The displayed information may be character string information or imageinformation indicating whether or not the candidate image is suitablefor a reference image, for example. Further, it is possible to displayone or more candidate images (or their thumb nails) processed in thejudgment by the candidate image judging part 234, and also display alist of information indicating the judgment results. Further, it ispossible to display a list of imaging information of candidate images inthe imaging information display of the patient selecting screendescribed in the first embodiment, and also display the judgment resultsin this list. In a case of carrying out such informing control, thedisplay controller 2111 functions as an example of an “informingcontroller”.

Informing control is not limited to such display control. For example,it may be configured that the controller 210 controls an audio outputpart (not illustrated) to output audio information indicating thejudgment result.

it is possible to prohibit follow up imaging based on a candidate imagewhen the judgment result that this candidate image is unsuitable for areference front image is obtained. As a specific example thereof, thecontroller 210 may prohibit optical scanning by the cross sectionalimage forming part in response to acquisition of the judgment resultthat the candidate image is not suitable for a reference front image.The prohibition here means a control mode in which OCT measurement isnot carried out even when an instruction for starting OCT measurement isinput. The controller 210 that performs such control is an example of a“prohibition controller”. Further, supposing cases in which follow upimaging is carried out by referring to a candidate image that has beenjudged as “unsuitable”, for example, a configuration may be employed inwhich an operation for canceling such prohibition can be done.

According to such an ophthalmologic imaging apparatus, it is possible tojudge whether a reference image used in follow up imaging is suitable,thereby carrying out follow up imaging with referring to a preferablereference front image. Therefore, the degree of an error in follow upimaging is evaluated quantitatively as in the first embodiment, and alsoprecision and accuracy of follow up imaging may be improved.

An ophthalmologic imaging apparatus may be configured to includefeatures of the second embodiment and not include features of the firstembodiment. Such an ophthalmologic imaging apparatus may have aconfiguration illustrated in FIG. 10, for example.

Specifically, this ophthalmologic imaging apparatus is capable ofcarrying out follow up imaging and includes a photographing part, across sectional image forming part, a storage, a candidate image judgingpart and an informing part. The photographing part (retinal camera unit2) photographs the eye and acquires a front image thereof. The crosssectional image forming part (optical system for OCT measurement and theimage forming part 220) scans the eye with light and forms a crosssectional image thereof. The storage (storage 212) stores one or morecandidate images for a front image referred to in follow up imaging. Thecandidate image judging part (candidate image judging part 234) analyzesthe candidate images and judges whether or not the candidate images aresuitable for a reference front image. The informing part performsinforming based on the judgment result from the candidate image judgingpart.

According to such an ophthalmologic imaging apparatus, it is possible tojudge whether a reference image used in follow up imaging is suitable,thereby carrying out follow up imaging with referring to a preferablereference front image. Therefore, accuracy of follow up imaging may beimproved. It should be noted that processing executed by the candidateimage judging part 234 may be the same as above. Further, it may beconfigured to carry out display control and informing control describedabove.

Third Embodiment

The present embodiment describes an ophthalmologic image processingapparatus that receives information from an ophthalmologic imagingapparatus capable of carrying out follow up imaging and processes thereceived information. The ophthalmologic image processing apparatus isconfigured to include a computer, for example. Further, part of theophthalmologic image processing apparatus may be arranged outside acomputer. For example, storage may be a database on a network.

FIG. 11 illustrates a configuration example of an ophthalmologic imageprocessing apparatus of the present embodiment. This ophthalmologicimage processing apparatus includes a configuration similar to the firstembodiment (see FIG. 4). On the other hand, this ophthalmologic imageprocessing apparatus does not include the photographing part (retinalcamera unit 2) and the cross sectional image forming part (opticalsystem for OCT measurement and the image forming part 220). Further, theophthalmologic image processing apparatus does not include computerprograms for controlling these excluded components. Hereinafter,descriptions of components similar to the first embodiment are omittedunless otherwise stated.

The ophthalmologic image processing apparatus of the present embodimentprocesses images acquired by follow up imaging and include at least astorage, an information obtaining part and a calculator. The storage(storage 212) stores a first front image of an eye and a second frontimage acquired in follow up imaging carried out with referring to thefirst front image. The information obtaining part (information obtainingpart 231) analyzes the first and second front images and obtainsmisregistration information between these front images. The calculator(calculator 232) calculates an evaluation value of an error in ascanning position of light in the follow up imaging based on themisregistration information. Configurations and operations of othercomponents correspond to the first embodiment.

According to such an ophthalmologic image processing apparatus, thedegree of the error in the follow up imaging carried out by anophthalmologic imaging apparatus can be evaluated quantitatively.

Modification Examples

The configuration described above is merely illustrations for favorablyimplementing the present invention. Therefore, it is possible to makearbitrary modification (omission, replacement, addition, etc.) withinthe scope of the present invention.

In the first and second embodiments, the optical path length differencebetween the optical path of the signal light LS and the optical path ofthe reference light LR is changed by varying the position of the opticalpath length changing part 41; however, a method for changing the opticalpath length difference is not limited to this. For example, it ispossible to change the optical path length difference by providing areference mirror (reference mirror) in the optical path of the referencelight and moving the reference mirror in the advancing direction of thereference light to change the optical path length of the referencelight. Further, the optical path length difference may be changed bymoving the retinal camera unit 2 and/or the OCT unit 100 with respect tothe eye E to change the optical path length of the signal light LS.Moreover, in a case that an object is not a living site or the like, itis also effective to change the optical path length difference by movingthe object in the depth direction (z-direction).

Computer programs for implementing the above embodiments can be storedin any kind of recording medium that can be read by a computer. As suchrecording media, for example, an optical disk, a semiconductor memory, amagneto-optic disk (CD-ROM, DVD-RAM, DVD-ROM, MO, and so on), and amagnetic storage (a hard disk, a floppy Disk™, ZIP, and so on) can beused.

In addition, it is possible to transmit/receive this program through anetwork such as internet or LAN etc.

EXPLANATION OF SYMBOLS

-   1 ophthalmologic imaging apparatus-   2 retinal camera unit-   10 illumination optical system-   30 imaging optical system-   31 focusing lens-   31A focus driver-   41 optical path length changing part-   42 galvano scanner-   50 alignment optical system-   60 focus optical system-   100 OCT unit-   101 light source unit-   105 optical attenuator-   106 polarization controller-   115 CCD image sensor-   200 arithmetic and control unit-   210 controller-   211 main controller-   2111 display controller-   212 storage-   2121 follow up imaging information-   220 image forming part-   230 image processor-   231 information obtaining part-   232 calculator-   2321 relative position calculator-   2322 judging part-   2323 area calculator-   2324 displacement calculator-   2325 evaluation value calculator-   233 imaging-propriety judging part-   234 candidate image judging part-   240A display-   240B operation part-   E eye-   Ef (eye) fundus-   LS signal light-   LR reference light-   LC interference light

What is claimed is:
 1. An ophthalmologic imaging apparatus capable of carrying out follow up imaging for acquiring a cross sectional image by referring to a front image of an eye acquired in the past and scanning the same position as before with light, comprising: a photographing part configured to photograph the eye and acquire a front image thereof; a cross sectional image forming part configured to scan the eye with light and form a cross sectional image thereof; a storage configured to store a first front image of the eye and a second front image acquired in follow up imaging carried out with referring to the first front image; an information obtaining part configured to analyze the first and second front images and obtain misregistration information between these front images; and a calculator configured to calculate an evaluation value of an error in a scanning position in the follow up imaging based on the misregistration information.
 2. The ophthalmologic imaging apparatus of claim 1, wherein the storage further stores a first scanning position that is a scanning position, in the first front image, of a cross sectional image formed associated with the first front image and a second scanning position that is a scanning position, in the second front image, of a cross sectional image formed associated with the second front image, the calculator comprises an area calculator configured to calculate an area of an image region defined by the first and second scanning positions based on the misregistration information, and the calculator calculates the evaluation value based on the area of the image region.
 3. The ophthalmologic imaging apparatus of claim 2, wherein the calculator comprises a judging part configured to judge, based on the misregistration information, whether or not the first and second scanning positions have a common position, and the area calculator calculates the area by executing different arithmetic processing according to the judgment result from the judging part.
 4. The ophthalmologic imaging apparatus of claim 3, wherein each of the first and second scanning positions is of a line-segment shape, the common position is an intersection point of these scanning positions of the line-segment shape, when it is judged that the intersection point exists, the area calculator calculates a sum of an area of a first triangle formed by the intersection point and end points of the first and second scanning positions located on one side from the intersection point and an area of a second triangle formed by the intersection point and end points of the first and second scanning positions located on the other side, and sets the calculation result of this sum as the area of the image region.
 5. The ophthalmologic imaging apparatus of claim 4, wherein when the intersection point is located at an endpoint of the first scanning position and/or an endpoint of the second scanning position, the area calculator obtains the area of the first triangle by calculating an area of a triangle formed by the intersection point and endpoints of the first and second scanning positions located on the opposite side to the intersection point, and sets the calculation result of this area as the area of the image region.
 6. The ophthalmologic imaging apparatus of claim 3, wherein each of the first and second scanning positions is of a line-segment shape, the common position is an intersection point of these scanning positions of the line-segment shape, when it is judged that the intersection point does not exist, the area calculator calculates an area of a quadrangle having the first and second scanning positions as two sides and line segments connecting endpoints of the first scanning position and endpoints of the second scanning position as other two sides, and sets the calculation result of this area as the area of the image region.
 7. The ophthalmologic imaging apparatus of claim 2, wherein in the processing for calculating the evaluation value from the area of the image region, the calculator executes arithmetic processing of subtracting a product of the area of the image region and a preset weight from a preset maximum of evaluation values.
 8. The ophthalmologic imaging apparatus of claim 2, comprising a first display controller configured to display the first and/or second front images on a display and displays a first scanning position image indicating the first scanning position and a second scanning position image indicating the second scanning position over the front images based on the misregistration information.
 9. The ophthalmologic imaging apparatus of claim 8, wherein the first display controller displays the first and second scanning position images in different aspects from each other.
 10. The ophthalmologic imaging apparatus of claim 1, wherein the storage further stores a first scanning position that is a scanning position, in the first front image, of a cross sectional image formed associated with the first front image and a second scanning position that is a scanning position, in the second front image, of a cross sectional image formed associated with the second front image, the calculator comprises a displacement calculator configured to calculate a displacement between the first and second scanning positions based on the misregistration information, and the calculator calculates the evaluation value based on the displacement.
 11. The ophthalmologic imaging apparatus of claim 10, wherein the displacement calculator calculates a displacement between a preset position in the first scanning position and a position in the second scanning position corresponding to this preset position.
 12. The ophthalmologic imaging apparatus of claim 11, wherein each of the first and second scanning positions is of a line-segment shape, the preset position includes at least one of a start point, end point and middle point of scanning in the scanning positions of the line-segment shape.
 13. The ophthalmologic imaging apparatus of claim 10, wherein in the processing for calculating the evaluation value from the displacement, the calculator executes arithmetic processing of subtracting a product of the displacement and a preset weight from a preset maximum of evaluation values.
 14. The ophthalmologic imaging apparatus of claim 1, wherein the storage further stores a first scanning position that is a scanning position, in the first front image, of a cross sectional image formed associated with the first front image and a second scanning position that is a scanning position, in the second front image, of a cross sectional image formed associated with the second front image, the calculator comprises: an area calculator configured to calculate an area of an image region defined by the first and second scanning positions based on the misregistration information; and a displacement calculator configured to calculate a displacement between the first and second scanning positions based on the misregistration information, and the calculator calculates the evaluation value based on the area of the image region and the displacement.
 15. The ophthalmologic imaging apparatus of claim 1, comprising a second display controller configured to display the evaluation value calculated by the calculator on a display.
 16. The ophthalmologic imaging apparatus of claim 1, comprising an imaging-propriety judging part configured to judge propriety of follow up imaging based on the evaluation value calculated by the calculator.
 17. The ophthalmologic imaging apparatus of claim 1, wherein the information obtaining part calculates a parallel shift error and rotational shift error between the first and second front images as the misregistration information.
 18. The ophthalmologic imaging apparatus of claim 1, wherein the storage stores, as the second front image, front images acquired first and last in the period in which light scanning is carried out in follow up imaging.
 19. The ophthalmologic imaging apparatus of claim 1, wherein when follow up imaging is carried out, the storage stores one or more candidate images for the first front image, and the ophthalmologic imaging apparatus comprises: a candidate image judging part configured to analyze the candidate images and judge whether or not the candidate images are suitable for the first front image; and an informing part configured to performs informing based on the judgment result from the candidate image judging part.
 20. The ophthalmologic imaging apparatus of claim 19, wherein the candidate image judging part executes judgment based on pixel values of the candidate images.
 21. The ophthalmologic imaging apparatus of claim 19, wherein the candidate image judging part calculates spatial frequency components of the candidate images and executes judgment based on the spatial frequency components.
 22. The ophthalmologic imaging apparatus of claim 19, wherein the informing part comprises an informing controller configured to display information indicating the judgment result on a display.
 23. The ophthalmologic imaging apparatus of claim 19, comprising a prohibition controller configured to prohibit light scanning by the cross sectional image forming part in response to acquisition of the judgment result that the candidate images are not suitable for the front image referred to.
 24. An ophthalmologic imaging apparatus capable of carrying out follow up imaging for acquiring a cross sectional image by referring to a front image of an eye acquired in the past and scanning the same position as before with light, comprising: a photographing part configured to photograph the eye and acquire a front image thereof; a cross sectional image forming part configured to scan the eye with light and form a cross sectional image thereof; a storage configured to store one or more candidate images for a front image referred to in follow up imaging; a candidate image judging part configured to analyze the candidate images and judge whether or not the candidate images are suitable for the front image referred to; and an informing part configured to performs informing based on the judgment result from the candidate image judging part.
 25. An ophthalmologic image processing apparatus that processes images acquired by follow up imaging for acquiring a cross sectional image by referring to a front image of an eye acquired in the past and scanning the same position as before with light, comprising: a storage configured to store a first front image of the eye and a second front image acquired in follow up imaging carried out with referring to the first front image; an information obtaining part configured to analyze the first and second front images and obtain misregistration information between these front images; and a calculator configured to calculate an evaluation value of an error in a scanning position of the light in the follow up imaging based on the misregistration information. 